Sleep complaints increase significantly with age in both men and women, but the increase is especially striking in women. Older women experience more nighttime awakenings, longer sleep onset latencies, and "lighter" sleep. Insomnia, disrupted sleep, and consequent daytime drowsiness are associated with an increased risk of accidents, increased utilization of health care and sedative-hypnotic medications, and a reduced quality of life. Older women receive a disproportionate number of prescriptions for sedative-hypnotics, which can exacerbate sleep apnea and lead to daytime carryover effects such as sedation, falls and subsequent fractures, and cognitive impairment. A better understanding of the sleep changes experienced by older women is sorely needed. One contributing factor may be age-related changes in sex steroids such as estrogen. The very low levels of estrogen that occur post-menopause have wide-ranging chronic effects, from increased cardiovascular risk factors to possible effects on memory. Sleep changes in older women may also be related to this dramatic change in hormonal milieu. Several studies have shown that ERT can improve the sleep of peri-menopausal women, and our preliminary data shows that the use of ERT is associated with better sleep in older post-menopausal women. Estrogen acts on several brain areas important for sleep and circadian rhythms (e.g., the suprachiasmatic nucleus, the hypothalamic pre-optic area, and the pineal gland). Previous studies of ERT effects on sleep were based on peri-menopausal women who were experiencing hot flashes and/or other menopausal symptoms including depression. All but a few of these studies were based on brief subjective sleep ratings. No published studies have examined the effect of ERT on the sleep of post-menopausal women., i.e., women who are several years past menopause, menopausal symptoms, and menopause-related hormone fluctuations. The proposed study will employ laboratory-based polysomnography and a randomized, placebo-controlled within-subjects design to assess the effect of six months ERT or placebo on the sleep and circadian rhythms of post-menopausal women. The results will help determine the role of age-related estrogen decline in the decreased sleep quality of older women. This study is the first step in a research program investigating the relationship between sex steroids and sleep in older individuals.